1. Field of the Invention
The present invention relates to an image heating apparatus for use in image forming apparatuses such as a copying machine and a printer, particularly to an image heating apparatus having a plurality of heaters.
2. Related Background Art
In an image forming apparatus using an electrophotographic system, during image formation, the surface of an electrophotographic photosensitive member is uniformly charged by a charge apparatus, the surface of the charged electrophotographic photosensitive member is exposed to light by an exposure apparatus, and an electrostatic latent image is formed. Subsequently, this electrostatic latent image is developed by a developing apparatus to form a toner image, this toner image is transferred to transfer materials such as a sheet by a transfer apparatus, and the toner image is fixed as a permanent fixed image on the transfer material by a fixing apparatus, and outputted.
In the fixing apparatus, the transfer material with the unfixed toner image transferred thereon is pinched and conveyed in a fixing nip portion between a fixing roller and a pressurizing roller, and the unfixed toner image is heated by a heater as a heating material disposed in the fixing roller, so that the unfixed toner image can be fixed as the permanent fixed image on the transfer material.
In the fixing apparatus, the temperature of the fixing roller is usually detected by temperature detecting elements such as a thermistor, and the electrical power supply to an inner heater is controlled so that the detected temperature of the temperature detecting element is maintained at a set temperature suitable for fixing a toner. By this temperature control, the fixing roller can be maintained at the set temperature even if heat is taken by paper during fixing.
Additionally, as one example of the fixing apparatus, two heaters (halogen heaters) of a main heater and a sub-heater are disposed in the fixing roller.
During the fixing, control is performed in order to constantly light the main heater and light the sub-heater only when the detected temperature of the temperature detecting element is lower than the set temperature, so that a temperature drop can be suppressed with a small power consumption even during continuous printing.
Additionally, in the conventional temperature control to the above-described heaters (main heater and sub-heater), when only the main heater is in a driven state, and the sub-heater starts to be driven, then the non-driven sub-heater is in a cool state.
Therefore, when the total of currents of a large rush current value flowing at a timing when the sub-heater is driven and a current value flowing through the main heater increases, a fluctuation of an input voltage of an alternating-current power source itself for supplying an electrical power to the heaters (main heater and sub-heater) increases, thereby causing a problem that a flicker increases.